After last week's nozzle burn out problem, I changed me assembly a little. I made a reamer to cut the ID of the 3/4" pipe out to the OD of the 7/8" PVC rod that I was using for nozzle stock. I also made a reamer to cut the divergence cone for the nozzle. With the PVC rod center drilled to the throat size (0.21"), I ran the reamer into the rod, using my drill press, to make the nozzle. I cut the nozzle off the rod, then drilled and sanded a convergence cone into it.

After firing the throat measured 0.31". In the photo of the nozzle you can see a spacer ring that the fuel sat on. The bottom of the spacer is flush with the shoulder made by the reamer that cut the ID of the casing out to 7/8". The nozzle piece was very tight in the case and was cemented against the shoulder/spacer for a very good seal. I also put two 1/4" thick disks in for a bulkhead and let them dry for a day. Then I cemented a retaining ring behind them for good measure. After firing, the engine showed no signs of leaks and no bulges. The total impulse was 21.1 lb*sec. With 70 grams of fuel that comes out to 137 S.I. Peak thrust was at 64.6 pounds, which was 575 on my 600 lb gauge. I am still dividing the gauge by 8.9 even though it looks like 8.75 is closer to right, when I put weights on the arm. So it could have peaked at 65+ pounds. At this point I think I am finished with design changes and am ready to try to launch something. Also attached is the curve of this engine (magenta), the one that had the nozzle leak (yellow) and the one with the copper insert in the throat (blue).